38 fountain been sitting for 10 years. What to expect?

It is not a good idea to try to start it until the raw water pumps are replaced, the plugs pulled and the bores looked at with a borascope, heat exchanger pulled and inspected, headers pressure tested, fuel tanks drained and cleaned, fuel injection system flushed and purged of all old gas, new batteries installed, and engines turned over by hand if the bores are rust free. Then and only then would I try to turn the key to "start".

First step is inspecting the bores, if they are all rusted up, have to pull the engines and go through them bit by tiny bit. The problem is that it will not be a simple rebuild of just the rotating bits, but the whole system, computers/injection/sensors could all be fubar. Then if you have to pay someone else to do all this work, you could be out 70 to 80 % of the cost of new drop in replacement engines. 525's are not cheap. If you do it all yourself, then it will still be expensive in parts and the 100's of hours of your time.
A running boat is worth two in the bush.

There are deals to be had with running, well maintained boats. Unless you have the facilities, the knowledge, the time, the coin, and the patience to mess with this nightmare, I'd look elsewhere for a deal. One of the hardest things on mechanical items is a lack of use.

Its worth the trailer it's sitting on + the cost of a 42 hull + the drives used. Then knock off about 25% and you get the real worth of her as she sits. I doubt the owner will go for it, but I wouldn't spend more than 15K. You will have 25-30K into it to have a good running boat if you do the work yourself and the motors just need a light freshening. If you need new power, you'll drop 50K to have a good solid boat and honestly you could spend more. Still worth it too you?

10yrs theres storage fees out the ass somewhere something dont add up, the dealer could get a title if he wanted, if it set at my shop i would had already got the paperwork and sold it or been using it with a clean title in hand. Something dont sound right.

I would expect Unforseen Headaches
I would expect to Spend 5 times what you think you will to get it running
I would expect it to take 3 times as long to get it running as you anticipate
I would expect to be underwater in it by the time you get done refurbishing it
I would expect to look back in a couple years and say to yourself "not my brightest decision"
I would expect that the owner has unrealistic expectations on price

Just buy it for $15K. Gut the sucker. Dump the engines on ebay for 15K. Keep the exhaust and pressure check it, buy some decent carbed engiens to put in it, new upholstery and guages. Re fresh the drives and have a nice turn key boat for under 40K.